The present invention is related to a method of preparing contacts on the surface of semiconductor substrates. The present invention is also related to products obtained by this method and more particularly to a solar cell.
Conventional screen printing is currently used in a mass scale production of solar cells. Typically, the top contact pattern of a solar cell consists of a set of parallel narrow finger lines and wide collector lines deposited essentially at a right angle to the finger lines on the semiconductor substrate or wafer.
Such front contact formation of crystalline solar cells is performed with standard screen printing techniques. It has advantages in terms of production simplicity, automation, and low production cost.
Low series resistance and low metal coverage (low front surface shadowing) are basic requirements for the front surface metallization.
According to the document Hybrid Circuit No. 30, January 1993, xe2x80x9cThick-film Fine-line Fabrication Techniquexe2x80x94Application to Front Metallization of Solar Cells,xe2x80x9d by A. Dziedzic, J. Nijs, and J. Szlufcik, minimum metallization widths of 100-150 xcexcm are obtained using conventional screen printing. This causes a relatively high shading of the front solar cell surface. In order to decrease the shading a large distance between the contact lines, i.e., 2 to 3 mm is required. On the other hand, this implies the use of a highly doped, conductive emitter layer. However, the heavy emitter doping induces a poor response of the solar cell to short wavelength light. Narrower conductive lines can be printed using ultra-thin stainless steel wire screens with a high mesh density of 325 or 400. A thin masking emulsion with a thickness of 5-15 xcexcm is required to produce a line definition on the screen of at least 50 xcexdm.
Although a line width of 50 xcexcm can be achieved, the line thickness decreases below 10 xcexcm measured after the firing process. This gives rise to increased line resistance causing high power dissipation, particularly in the main collector lines.
The fact that the fingers are ultra-thin can result in the interruption of such fingers.
Another main disadvantage of the ultra-thin screens is their higher cost and lower durability and/or reliability.
An alternative technique to the standard screen printing is the application of an etched or electroformed metal mask. The manufacturing process of such mask involves etching of a cavity pattern on the one side of the metal foil and a mesh pattern on the reverse side. Photoresist masking and precise mask positioning are necessary for double-sided etching of the metal foil. This implies a complicated design and a very high screen cost.
In the case of conventional wire mesh screens as well as in the case of the metal etched screens, the open area (mesh openings) is usually not higher than 50% of the pattern. The open area defines the maximum amount of paste transferred to the substrates and at the same time the wet line thickness. Another important point is that a small mesh aperture requires utilization of special inks formulated for fine line printing. This is in conflict with most of the commercially available silver pastes for solar cell front contact metallization. Silver powder has a tendency to create agglomerates of particles in the paste. In addition, a flake-shaped silver powder, usually used in the paste formulation for a solar metallization, increases the tendencies to create agglomerates of particles in the paste.
The modern solar cell processing includes growing of thin thermal oxide (50-250 xc3x85) on the top emitter surface using methods well known in microelectronics. Such an oxide layer passivates defects and recombination centers always present on the semiconductor surface. This process leads to an improvement of cell response to solar short wavelength radiation that in effect gives rise to a higher cell efficiency. Although commercially available screen printed pastes produce good contact to non-oxidized silicon surfaces, the firing through thermal oxide gives difficulties in obtaining high quality contacts with low resistance.
It should also be noted that the solar cell manufacturing process includes in most cases a step of applying an antireflection (AR) coating which can be deposited before or after the contact formation. If the AR layer is deposited before contact printing, it often gives rise to the problem of high contact resistance between silicon and printed contacts. This problem occurs particularly when silicon nitride is used as an antireflection coating.
If an AR layer is deposited after the contact formation, another problem is raised which is the soldering of the collector lines during the module fabrication.
The solution to this problem brings the xe2x80x9cfiring throughxe2x80x9d method described in PCT Document WO 89/12312, wherein the authors apply the commercially available silver paste xe2x80x9cFerro #3349xe2x80x9d to xe2x80x9cfire throughxe2x80x9d a silicon nitride ARC. A xe2x80x9cfired throughxe2x80x9d TiO2 AR layer is described in the paper by Nunoi, et al. xe2x80x9cHigh performance BSF silicon solar cell with fired through contacts printed on AR coatingxe2x80x9d, 14th IEEE PV Specialists Conferencexe2x80x941980, San Diego, USA, pp. 805-810.
PCT Document WO 92/22928 describes a solar cell and a method to make it wherein an antireflective coating is deposited on a semiconductor substrate before a first set of narrow elongated parallel electrodes are printed thereon and wherein finally a second set of elongated electrodes are affixed to each of the first electrodes.
It should be noted that the paste or the ink used in order to form the array of narrow elongated parallel electrodes is such that it penetrates said antireflective material and forms mechanically adherent and low electrical resistance contact with the front surface of the semiconductor substrate. This means that not all the conventional pastes can be used. Furthermore, in order to have such good contact between the semiconductor substrate and the narrow elongated parallel electrodes, a step of xe2x80x9cfiring throughxe2x80x9d is necessary.
The firing at the same time through the thermally grown silicon dioxide and antireflection coating (particularly silicon nitride) layers, although described in the technical literature, usually gives problems of high contact resistance and is difficult to achieve with commercial pastes.
E.P.O. Document EP-A-0002550 describes a method of forming a contact configuration for soldering a metal connection on a region of the surface of a semiconductor body comprising the provision by serigraphy, on at least a part of said region, of a conductive paste which comprises at least a principal metal, said paste then being vitrified thermally such that the dopant migrates into at least a surface part on the region of a surface of the semiconductor body.
The present invention has an object to provide improved semiconductor devices such as solar cells which do not have the drawbacks of the prior art.
More particularly, the present invention aims to form semiconductor devices such as solar cells wherein the electrical contacts exhibit a low series of resistance and a low metal coverage which also provides a low front surface shadowing.
Many other advantages will be mentioned hereunder in the description of the main characteristics of the present invention.
The present invention provides a method of forming the top contact pattern of a solar cell, which consists of a set of parallel narrow finger lines and wide collector lines deposited essentially at the right angles to the finger lines on a semiconductor substrate, characterized in that it comprises the following steps:
(a) screen printing and drying the set of narrow finger lines;
(b) printing and drying the wide collector lines on top of the set of finger lines in a subsequent step;
(c) firing both finger lines and collector lines in a single final step in order to form an ohmic contact between the finger lines and the semiconductor substrate and between the finger lines and the wide collector lines.
According to a first preferred embodiment, the following steps are performed before the screen printing step of the contact finger lines:
(1) screen printing a pattern of masking paste on the front surface of the semiconductor substrate, so that the printed pattern will form the pattern for the set of parallel finger lines;
(2) depositing an antireflection coating over the whole front surface;
(3) dissolving the masking paste and selectively lifting-off the portions of the antireflection coating which have been deposited on the masking paste;
(4) etching-off the oxide layers from the openings in the antireflection coating;
(5) performing the steps (a), (b), and (c) as described hereabove.
According to another possible embodiment of the present invention, an antireflection coating is deposited in an intermediate step after printing and drying the front contact finger lines and before the collector lines are printed and dried.
The several methods described hereabove can be applied to substrates already having a rear ohmic contact or a back contact can be formed during the front contact formation or after the front contact has been already fabricated.
It should be noted that according to the method of the present invention, the last step is only a co-firing step and not a step of firing through.
The screen for printing the set of narrow parallel finger lines is preferably made from a solid metal foil in which the set of parallel lines which form the finger contact pattern can be chemically etched or cut by a laser or an electron beam.
However, in some particular embodiments wherein bridges over the openings are allowed, other masks besides metal stencils can be used, such as a mesh screen.
The screen used for printing the collector lines is preferably made of a conventional mesh screen or a metal stencil screen.
Other techniques such as ink-jet printing or off-set printing can also be used in the present invention for printing the collector lines. The proposed invention results in many advantages over using conventional screen printing techniques.
Concerning Finger Lines
1. The pattern of parallel finger lines is formed in a solid metal foil which means that it has an open area equal to 100%. No meshes are present in the pattern openings. This increases the volume of the paste transferred to the substrate in the printing process. It should be noted that when using a standard wire mesh screen, the open area is only between 40%-60%.
2. The absence of the meshes in the openings reduces the requirements for good screen printability of pastes used for front contact printing. Pastes with a high solids content and high viscosity can be used.
3. Using a laser or an electron or ion beam for metal mask cutting gives the possibility of obtaining pattern definition down to a few micrometers. This depends on the metal foil thickness and the quality of the cutting system. In practice, the line width is limited by the requirement of high line thickness. The thicker the metal mask the higher is the thickness of the printed lines. On the other hand, the ratio of the cut line width to the mask thickness should be above 0.5. A lower ratio leads to difficulties in paste transfer through the mask openings during printing. It has been demonstrated that laser cutting can fabricate a finger pattern of 30 xcexcm wide lines cut in a 50-60 xcexcm thick stainless steel foil.
The result of the above advantages 1-3 gives the possibility of printing very narrow lines with a high aspect ratio and no interruption. Lines with width of 40 xcexcm and up to 25-30 xcexcm thickness have been measured after printing and drying. This corresponds to 13-16 xcexcm thick lines after firing. A metal sheet resistance of 1-2 mohm/sq. was measured with most commercially available specialized pastes for solar cell front contacts.
4. Furthermore, using solid stainless steel stencils instead of wire mesh screens for printing the finger lines increases the durability of the screens.
5. Cutting the continuous and completely open lines by a laser or electron beam simplifies the screen fabrication process and strongly decreases the screen cost.
Concerning Collector Lines
1. A collector pattern is preferably prepared with a conventional wire mesh screen or with solid metal masks. A durable screen with a total (screen+emulsion) thickness above 100 xcexcm can be used. A standard screen with a mesh density of 200 or 180 per inch covered with a 20 xcexcm thick emulsion is typical for collector printing.
2. The thick collector lines with a sheet resistance below 1 mohm/sq. will be easily obtained with most of the silver pastes for front contact metallization. The width of the collector lines can be decreased, giving lower shading. Using the preferred embodiment of the present invention wherein prior to the screen printing of the finger lines, a screen printing of a masking paste is performed with the deposition of antireflection coating, the following advantages can be noted:
Concerning the Masking Paste
The role of the masking paste is to provide a selective mask for an antireflection coating (ARC) deposition at those regions of oxidized silicon substrates where the front contact finger lines are going to be printed. The masking paste after the drying or curing process should stay intact during the ARC deposition and be easily removed, later lifting off of the ARC layer deposited on top of it. Pastes containing fine metal powders or powders of silicon oxide, titanium oxide, or chalk powder mixed with an organic vehicle fulfill the task. These pastes are easily removed in organic solvents.
Concerning the Front Finger Contact Paste
Since there is no intermediate layer between the printed front finger contact and silicon substrate and since the applied laser cut stencil screens have no blocking meshes, the requirements for the front contact silver pastes specialized for the front contact formation can be applied in the present invention.
Concerning the Front Collector Paste
The paste applied for the collector lines can be the same silver paste as for the front fingers or any other high conductivity paste which gives a good adherence to an antireflection coating layer, does not penetrate completely through the ARC, and provides a perfect low resistance ohmic contact to finger lines.
Furthermore, in the case when an antireflection coating is used, the following advantage could be mentioned:
1. Both finger and collector lines are co-fired in the same firing process. As a result, the finger lines are in good electrical contact with the substrate, and the collector lines are in good electrical contact with the finger lines. In any case, the collector lines are not covered by an ARC layer. This gives no problem with soldering of collector lines during module fabrication.
2. Separation of the collector lines from the direct contact with silicon substrate reduces carrier recombination losses existing at the metal contact-silicon interface. Selection of material used for the ARC coating and of the deposition technique are crucial for achieving a separation. Most top contact silver pastes penetrate through an ARC layer of titanium dioxide deposited by Atmospheric Pressure Chemical Vapor Deposition (APCVD). In the case of using a silicon nitride AR layer deposited by Plasma Enhanced CVD, such ARC layer can be a very good barrier between silicon and most screen printed silver pastes.
3. Solar cell contacts prepared according to the present invention can have contact finger lines placed much more closely without additional shadowing. Solar cells with lightly doped emitter and a higher sheet resistance can be fabricated by a screen printing solar cell process, which results in an improvement of solar cell response to short wavelength light.
Accordingly, the solar cells having electrical contacts prepared with the method according to the present invention exhibit:
low sheet resistance of fingers;
lower sheet resistance of collectors;
lower contact resistance of finger/substrate interface;
lower series of solar cell;
lower shadowing losses caused by fingers;
lower shadowing losses caused by collector lines;
lower solar cell total shadowing losses;
lower carrier recombination losses at contact-silicon interfaces.